Synthesis of Atomically Flat La_2-xSr_xCuO₄ Superconducting Thin Films by Atomic-Layer-by-Layer Molecular Beam Epitaxy

The molecular beam epitaxy technique allows us to synthesise atomically smooth single-crystal thin films, the study of which triggers progress in condensed-matter physics and materials science. We present a method of growth of atomically perfect thin films of La_2-xSr_xCuO₄ (LSCO), the archetypical high-temperature copper oxide superconductor, using a custom-built molecular beam epitaxy system. We first describe the steps prior to the film growth, such as substrate preparation and calibration of fluxes from elemental sources. We then describe the atomic-layer-by-layer growth mode and the post-annealing procedures appropriate for different doping levels. We further illustrate and discuss the strategy to monitor the film growth and maintain the correct stoichiometry, based on real-time feedback from the Reflection High Energy Electron Diffraction (RHEED). The atomic force microscopy shows that the films are atomically smooth without any secondary phase precipitates. Magnetic measurements show that T_c is homogeneous to within 0.1 K over the entire film area.